Book : Cell Biology
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Posted by: CHELSEA
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Writer CHELSEA

Mitosis

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Mitosis is a type of cell division in which one cell (the mother) divides to produce two new cells (the daughters) that are genetically identical to it. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes. The great majority of the cell divisions that happen in your body involve mitosis. During development and growth, mitosis populates an organism’s body with cells, and throughout an organism’s life, it replaces old, worn-out cells with new ones. For single-celled eukaryotes like yeast, mitotic divisions are actually a form of reproduction, adding new individuals to the population.

In all of these cases, the “goal” of mitosis is to make sure that each daughter cell gets a perfect, full set of chromosomes. Cells with too few or too many chromosomes usually don’t function well: they may not survive, or they may even cause cancer. So, when cells undergo mitosis, they don’t just divide their DNA at random and toss it into piles for the two daughter cells. Instead, they split up their duplicated chromosomes in a carefully organized series of steps.

Interphase:
  • Mitosis alternates with interphase. In interphase the cell prepares itself for division. 
  • The interphase is divided into three phases G1, S and G2. 
  • During these phases the cells grow by producing proteins and cytoplasmic organelles. 
  • In the S phase the chromosomes replicate. In G1 phase the cell grows, in S phase the chromosomes duplicate and in G phase the cell grows more and prepares for mitosis and finally divides in the Mitotic cycle. 
  • The cell cycle is regulated by proteins.
  • The phases of the interphase follow strict order and have checkpoints. 
  • There is another phase in the interphase G0 where the cell has the option to enter this stage.
  • Interphase takes about 90% of the cell's life span. 

Prophase:
  • In the nucleus the genetic material is loosely bundled in coil called chromatin. 
  • At the onset of prophase the chromatin fibres become tightly coiled and condense into discrete chromosomes. 
  • Inside the nucleus, the nucleolus also disappears from view. 
  • The centrioles begin to move to opposite ends of the cell and the spindle fibres extend from the centromere.
  • Some fibres cross the cell to form the mitotic spindle fibres. 

Prometaphase:
  • Prometaphase is sometimes considered as the end of prophase and early metaphase.
  • During the early stage of prometaphase the nuclear membrane disintegrates and the microtubules enter the nuclear space. 
  • This is known as "open mitosis" and it occurs in most multicellular organisms.
  • Organisms like fungi, some portists like algar or trichomonads undergo "closed mitosis" where the spindle formation happens inside the nucleus. 
  • The nuclear membrane stays intact and the microtubules are not able to penetrate the intact nuclear membrane. 
  • During the late prometaphase, at the centromere of each chromosome forms two kinetochores.
  • Kinetochore is a complex protein structure, it is the point where the microtubules attach themselves to the chromosome. 

Metaphase:
  • The term metaphase is derived from Greek word 'meta' which means 'after'.
  • In the prometaphase after the microtubules are attached to the prometaphase the chromosomes start pulling the chromosomes towards the ends of the cell. 
  • The centromeres of the chromosomes assemble along the metaphase plate also known as the equatorial plane.
  • It is an imaginary line that is in between the centrosome poles and is called the spindle equator.
  • This helps to ensure that when the chromosomes are separated the new nucleus will receive one copy of each chromosome. 
 
Anaphase:
  • After the metaphase stage the chromosomes proceed to the anaphase stage. 
  • The term anaphase is derived from the Greek word "ava" which means "up", or "against", or "back", or "re".
  •  First the proteins that bind the sister chromatids are cleaved making the sister chromatids as separate daughter chromosomes and are pulled apart towards the respective centrosomes to which they are attached. 
  • The microtubules at the poles pull the set of chromosome that are attached to it the opposite ends of the cell. At the end of anaphase the microtubules all degrade. 
 
Telophase:
  • Telophase is derived from the Greek word "telos" meaning "end". 
  • It is a reversal of prophase and prometaphase events. In the telophase stage the polar microtubules continue to lengthen elongating the cell. 
  • The daughter chromosomes attach at opposite site ends of the cell.
  • New membranes are formed around the daughter nuclei.
  • The chromosomes spread and are no longer visible under the light microscope. 
  • The spindle fibers also disperse; cytokinesis may also begin during this stage. 
 
Cytokinesis:
  • Cytokinesis is a separate process that begins at the same time as the telophase. 
  • Cytokinesis is not a phase of mitosis, it is a separate process necessary for completing cell division.
  • In animal cells a pinch like cleavage furrow containing a contractile ring develops at the position of the metaphase plate separating the nuclei.
  • In the animal and plant cells the division of cell is driven by vesicles derived from the Golgi apparatus.
  • In plant cells, the rigid wall requires a cell plate be synthesized between the two daughter cells. 
 
Significance of Mitosis:
  • Mitosis is an equational division.
  • It is the division through which daughter cells that are identical are produced.
  • The daughter cells have the same amount and type of genetic constitution as that of the parent cell. 
  • Mitosis division is responsible for growth and development of a single-celled zygote into a multicellular organism. 
  • The chromosome number remains the same in the cells produced by this division. 
  • The daughter cells have the same characters as those of the parent cell.
  • Mitosis division helps in maintaining the proper size.
  • Mitosis also helps in restoring wear and tear in body tissues, replacing damaged or lost part, healing wounds and regeneration of detached parts. 
  • This method of multiplication is seen in unicellualr organisms. 
  • Mitotic division of cell is unchecked and it may result in uncontrolled growth of cells leading to cancer or tumor.      

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